Use Rust 2021 edition for pre-commit's rustfmt

[david-perez]

In #1268, runtime crates were updated to Rust 2021 edition.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[github-actions]

A new generated diff is ready to view.

• AWS SDK (ignoring whitespace)
• No codegen difference in the Server Test

A new doc preview is ready to view.

Rust Wrk benchmark report:

Duration: 90 sec, Connections: 32, Threads: 2

Measurement	Deviation	Current	Old
Requests/sec	-20.89%	56726.17	71709.25
Total requests	-20.93%	5108934	6460944
Total errors	NaN%	0	0
Total successes	-20.93%	5108934	6460944
Average latency ms	-41.12%	0.63	1.07
Minimum latency ms	0.00%	0.02	0.02
Maximum latency ms	-21.99%	16.5	21.15
Stdev latency ms	-66.48%	0.6	1.79
Transfer Mb	-20.93%	531.08	671.62
Connect errors	NaN%	0	0
Read errors	NaN%	0	0
Write errors	NaN%	0	0
Status errors (not 2xx/3xx)	NaN%	0	0
Timeout errors	NaN%	0	0

[Velfi]

❤️ Thanks for catching this, sorry I missed it

[github-actions]

A new generated diff is ready to view.

• AWS SDK (ignoring whitespace)
• No codegen difference in the Server Test

A new doc preview is ready to view.

Rust Wrk benchmark report:

Duration: 90 sec, Connections: 32, Threads: 2

Measurement	Deviation	Current	Old
Requests/sec	-6.60%	77449.29	82920.84
Total requests	-6.61%	6974650	7468024
Total errors	NaN%	0	0
Total successes	-6.61%	6974650	7468024
Average latency ms	-13.08%	0.93	1.07
Minimum latency ms	0.00%	0.02	0.02
Maximum latency ms	-10.24%	21.22	23.64
Stdev latency ms	-18.42%	1.55	1.9
Transfer Mb	-6.61%	725.02	776.3
Connect errors	NaN%	0	0
Read errors	NaN%	0	0
Write errors	NaN%	0	0
Status errors (not 2xx/3xx)	NaN%	0	0
Timeout errors	NaN%	0	0